Plants and dinosaurian herbivores

As in most extant terrestrial mammalian communities, the majority of dinosaurs were herbivorous. If dinosaurs were numerous enough, and their impact on terrestrial ecosystems was important enough, there ought to be some relationship between herbivorous dinosaur evolution and plants.

Plants

Most paleobotanists - people who study extinct plants - recognize two major groupings of Mesozoic plants. The first is a non-monophyletic cluster of plants including ferns, lycopods, and sphenopsids (Figure 13.8). All of these plants tend to be low growing and primitive, but, like most land plants, they are vascular; that is, they possess specialized tissues that conduct water and nutrients throughout the plant.

The second major grouping of plants consists of gymnosperms and angiosperms. Together these two groups are united by the diagnostic character of possessing a seed (Figure 13.9). Seeds are ultimately nutrient-bearing pods apparently developed for the dissemination of gametes. Gymnosperms are today best known as pines and cypress, and

a lesser-known but Mesozoic-ly important group known as cycadophytes: plants with a pineapple-like stem and bunches of leaves springing out of their tops. Angiosperms today consist of magnolias, maples, grasses, roses, and orchids, among many other groups (Figure 13.9).

Several qualities distinguish these plant groups. In general, Mesozoic gymnosperms tended to be of three types: conifers, cycadophytes, and gingkoes. Conifers - epitomized, for example, by pines - were very tall and woody plants. They had relatively little nutritive value pound for pound, possessing coarse thick bark and cellulose-rich leaves. The modern representatives of these plants tend to secrete a variety of ill-tasting or poisonous compounds as a strategy to discourage their consumption; there is no reason to suppose that their Mesozoic counterparts were any different.

Cycadophytes, on the other hand, tended to be fleshier and softer, with perhaps more nutritive value. Gingkoes would also have been plants available for dinosaur consumption, and circumstantial evidence suggests they too were eaten by Mesozoic herbivorous dinosaurs (Figure 13.9).

Flowering plants evolved an entirely different approach to life from gymnosperms. Far from discouraging herbivores from consuming them, they evolved a variety of strategies to actively court their consumption by herbivores: bright tasty flowers, fruits with tough seeds that can survive a trip through a digestive tract. Consumption by herbivores in the

Figure 13.9. Representative cycads, gingkoes, gymnosperms, and angiosperms from the Mesozoic. (1) Cycadeoids or bennettitaleans, as they are sometimes called (Triassic-Cretaceous); (2 and 5) W\W\amson\eWa spp. (Triassic-Jurassic); (3) W\e\andd\eWa (Jurassic); (4) W\W\amson\eWasewardiana (Jurassic). Gingko: (6) Gingkoites (Triassic-Recent). Conifers: (7) Sequoia (mid Cretaceous-Recent); (8) Araucaria (Late Triassic-Recent); (9) PagiophyWum (Triassic-Cretaceous). Angiosperms: (10) Magnoliaceae (magnolias; still small-flowered in the early days of their appearance on earth; Cretaceous?-Recent); (11) Nymphaeaceae (water lilies; Late Cretaceous-Recent). Inset: Seed (dicot) in cross-section. The cotyledons, shoot apex, root apex, and suspensor are all parts of the embryonic plant. The endosperm is a food source for the embryo as it develops, and the seed coat protects the embryo and its food source.

Figure 13.9. Representative cycads, gingkoes, gymnosperms, and angiosperms from the Mesozoic. (1) Cycadeoids or bennettitaleans, as they are sometimes called (Triassic-Cretaceous); (2 and 5) W\W\amson\eWa spp. (Triassic-Jurassic); (3) W\e\andd\eWa (Jurassic); (4) W\W\amson\eWasewardiana (Jurassic). Gingko: (6) Gingkoites (Triassic-Recent). Conifers: (7) Sequoia (mid Cretaceous-Recent); (8) Araucaria (Late Triassic-Recent); (9) PagiophyWum (Triassic-Cretaceous). Angiosperms: (10) Magnoliaceae (magnolias; still small-flowered in the early days of their appearance on earth; Cretaceous?-Recent); (11) Nymphaeaceae (water lilies; Late Cretaceous-Recent). Inset: Seed (dicot) in cross-section. The cotyledons, shoot apex, root apex, and suspensor are all parts of the embryonic plant. The endosperm is a food source for the embryo as it develops, and the seed coat protects the embryo and its food source.

case of angiosperms appears to be a strategy for seed dispersal, not the destruction of the plant.

Dinosaurs and plants

Our analysis is built around Figure 13.10, which compares the record of Late Triassic through Late Cretaceous plant diversity with that of dinosaurian herbivores. The lower part of the figure gives approximations of the global composition of plants through the time of the dinosaurs. The upper part of the figure is divided into various groups of herbivorous dinosaurs.

Plants. In terms of plants, Figure 13.10 shows some key patterns. Lycopods, seed ferns, sphe-nopsids, and ferns decrease in global abundance during the Late Triassic interval. From then until the end of the Mesozoic, they constitute a roughly constant proportion of the world's floras. Not so with the gymnosperms, which dramatically increase their proportion of the total global flora during the Late Triassic. And it is clear that much of that increase is taken

Figure 13.10. Comparison of changes in plant diversity and herbivorous dinosaur diversity during the Late Triassic through Late Cretaceous time interval. Upper part of the diagram shows diversity of major groups of herbivorous dinosaurs through time. Comparison between this diagram and that shown in Figure 13.2 suggests that one of the most important things driving dinosaur evolution and diversity was the development of new (or improved) ways to exploit the environments in which they lived (see the text).

up by conifers, which constitute around 50% of the world's total floras throughout the rest of the Mesozoic.

Our best guess is that angiosperms first evolved in the very early part of the Cretaceous; however, it was during mid-Cretaceous times that they underwent a tremendous evolutionary burst. The uniquely efficient angiosperm seed dispersal mechanisms afforded by flowers were (and are) unparalleled in the botanical world, and consequently flowering plants have blossomed as no other group of plants has.

Co-evolution. What is the relationship between dinosaurian herbivores and plants? It cannot be purely by chance that the rise of tall coniferous forests is coincident with the appearance on Earth of the world's first tall herbivores: prosauropods (and later sauropods). Here we see the mark of co-evolution, the evolution of one group affecting - and even effecting - the evolution of another. In this case, it's plants and dinosaurs: were those tall prosauropods favored by natural selection that could take advantage of comparatively succulent leaves at the tops of conifers? Alternatively, were conifers that were particularly tall favored by natural selection in response to the increasing height of prosauropods? Which is cause and which is effect is something we'll likely never know.

The figure also reveals another compelling relationship. The rise of the angiosperms occurs at approximately the same time as several major radiations of dinosaur groups. Were these groups - ceratopsians, pachycephalosaurs, hadrosaurids, and late-evolved ankylo-saurs - groups that somehow took advantage of angiosperms as a food source and diversified? Is this a clue to what these dinosaurs were eating (Box 13.3).

It appears that, during the middle of the Mesozoic, few vertebrates fed very selectively upon the relatively slow growing conifers, cycads, and gingkoes that formed the majority of terrestrial floras. Instead, it has been suggested, dinosaur feeding consisted of low browsing and was rather generalized (similar to the way a lawn-mower "grazes" over whatever is in its path). Because so many of these Mesozoic herbivores were also very large and may have lived in large herds, they likely cleared expansive areas, trampling, mangling, uprooting, and otherwise disturbing areas that otherwise might be colonized by plants.

Such low-level, generalized feeding and disturbances of habitats tended to emphasize fast growth in plants, but discouraged the establishment of seed-dispersal relationships between plants and animals. Thus the picture of Mesozoic plant-herbivore interactions appears to be one in which (a) plants produced vast quantities of offspring to ensure the survival of the family line into the next generation and (b) herbivores took advantage of the rapidly and abundantly reproducing resource base to maintain their large populations of large individuals. Plant-herbivore co-evolution during the Mesozoic appears to have been based on habitat disturbance, generalized feeding, and rapid growth and turnover among plants.

To date, "mummified" remains2 of hadrosaurids (Edmontosaurus and Corythosaurus) do not show the remains of angiosperms in the digestive tract, but rather the remains of coniferous plants. Late Cretaceous coprolites, reliably attributed by size to either ceratopsids or hadrosaurids, contained conifer fragments as well. If angiosperms were fueling this dinosaur radiation, where are the angiosperm pieces that we might hope to find?

Yet dinosaur chewing efficiency increased markedly through the latter part of the Mesozoic. This is not to say that non-chewing dinosaurs were in a state of decline; as Figure 13.10 shows, sauropods - for whom chewing was a minimalist artform - were successful throughout the Cretaceous. Moreover, animals that indulged in rudimentary chewing - such as ankylosaurs and pachycephalosaurs - underwent strong evolutionary bursts during the latter part of the Cretaceous. Still, ceratopsids and hadrosaurids - groups that elevated chewing to new heights - are characteristic of the Late Cretaceous radiation. Did advanced chewing mechanisms allow hadrosaurids and ceratopsids to take advantage of food resources not heretofore available to other dinosaurs?

2. These rare and spectacular fossils are not truly mummified, which would mean that their original tissue is preserved (as is the case with the famous mummies of ancient Egypt). Rather, in this case, the original animal dried out (like a mummy) and was buried by sediment intact. Then, post-burial, all the original organic tissue was replaced by minerals resulting in, as discussed in Chapter 1, a perfect natural forgery of the original dessicated carcass (including soft tissue such as skin impressions, tendons, and stomach contents).